Spring preload systems are applied to parallel-axis differentials to create a resistance to differentiation. An advantage of this is having the differential apply a torque to both wheels until a differentiation threshold is exceeded. This allows light torque to reach one of a pair of driven wheels, even when its mate has lost traction.
Preloads for this purpose have been applied to parallel-axis differentials by spring washers, but these are problematic since the force of spring washers varies considerably with small changes in axial dimensions. Coil springs have been suggested for preloading parallel-axis differentials, as proposed by Saari in U.S. Pat. No. 3,375,736. Coil springs work better for preload purposes, because they are less sensitive to variation in axial dimensions.
Some cars and trucks require parallel-axis differentials with thrust blocks interposed between the ends of axles splined to side gears of the differentials. This is necessary to provide access for attaching C-clips holding ends of the axles within the differential. The necessary access and the presence of a thrust block between axle ends within a differential requires that planetary gears be arranged beyond the perimeter of the thrust block. With a central space between the axle ends occupied by a thrust block, and the planetary gears being crowded into sectors beyond the perimeter of the thrust block, there remains no room for any of the previously suggested ways of applying a preload to a parallel-axis differential.